The proton exchange membrane or polymer electrolyte membrane fuel cell (PEM-FC) has been a topic of highly active R&D efforts during the past two decades. The operation of a fuel cell normally requires the presence of an electrolyte and two electrodes, each comprising a certain amount of catalysts, hereinafter referred to as electro-catalysts. A hydrogen-oxygen PEM-FC uses hydrogen or hydrogen-rich reformed gases as the fuel while a direct-methanol fuel cell (DMFC) uses methanol solution as the fuel. The PEM-FC and DMFC, or other direct organic fuel cells, are collectively referred to as the PEM-type fuel cell.
A PEM-type fuel cell is typically composed of a seven-layered structure, including a central polymer electrolyte membrane for proton transport, two thin electro-catalyst layers on the two opposite sides of the electrolyte membrane in which chemical reactions occur, two gas diffusion layers (GDLs) or electrode-backing layers stacked on the corresponding electro-catalyst layers, and two flow field plates stacked on the GDLs. Each GDL normally comprises a sheet of porous carbon paper or cloth through which reactants and reaction products diffuse in and out of the cell. The flow field plates, also commonly referred to as bipolar plates, are typically made of carbon, metal, or composite graphite fiber plates. The bipolar plates also serve as current collectors. Gas-guiding channels are defined on a surface of a GDL facing a flow field plate, or on a flow field plate surface facing a GDL. Reactants and reaction products (e.g., water) are guided to flow into or out of the cell through the flow field plates. The configuration mentioned above forms a basic fuel cell unit. Conventionally, a fuel cell stack comprises a number of basic fuel cell units that are electrically connected in series to provide a desired output voltage. If desired, cooling and humidifying means may be added to assist in the operation of a fuel cell stack.
Several of the above-described seven layers may be integrated into a compact assembly, e.g., the membrane-electrode assembly (MEA). The MEA typically includes a selectively permeable polymer electrolyte membrane bonded between two electrodes (an anode and a cathode). A commonly used PEM is poly (perfluoro sulfonic acid) (e.g., Nafion® from du Pont Co.), its derivative, copolymer, or mixture. Each electrode typically comprises a catalyst backing layer (e.g., carbon paper or cloth) and an electro-catalyst layer disposed between a PEM layer and the catalyst backing layer. Hence, in actuality, an MEA may be composed of five layers: two catalyst backing, two electro-catalyst layers, and one PEM layer interposed between the two electro-catalyst layers. Most typically, the two electro-catalyst layers are coated onto the two opposing surfaces of a PEM layer to form a catalyst-coated membrane (CCM). The CCM is then pressed between a carbon paper layer (the anode backing layer) and another carbon paper layer (the cathode backing layer) to form an MEA. It may be noted that, some workers in the field of fuel cells refer a CCM as an MEA. Commonly used electro-catalysts include noble metals (e.g., Pt), rare-earth metals (e.g., Ru), and their alloys. Known processes for fabricating high performance MEAs involve painting, spraying, screen-printing and hot-bonding catalyst layers onto the electrolyte membrane and/or the catalyst backing layers.
An electro-catalyst is needed to induce the desired electrochemical reactions at the electrodes or, more precisely, at the electrode-electrolyte interfaces. The electro-catalyst may be a metal black, an alloy, or a supported metal catalyst, for example, platinum supported on carbon. In real practice, an electro-catalyst can be incorporated at the electrode-electrolyte interfaces in a PEM fuel cell by depositing a thin film of the electro-catalyst on either an electrode substrate (e.g., a surface of a carbon paper-based backing layer) or a surface of the membrane electrolyte (the PEM layer). In the former case, electro-catalyst particles are typically mixed with a liquid to form a slurry (ink or paste), which is then applied to the electrode substrate. While the slurry preferably wets the substrate surface to some extent, it must not penetrate too deeply into the substrate, otherwise some of the catalyst will not be located at the desired membrane-electrode interface. In the latter case, electro-catalyst particles are coated onto the two primary surfaces of a membrane to form a catalyst-coated membrane (CCM). The slurry, ink, or paste is hereinafter referred to as a precursor electro-catalyst composition.
Electro-catalyst sites must be accessible to the reactants (e.g., hydrogen on the anode side and oxygen on the cathode side), electrically connected to the current collectors, and ionically connected to the electrolyte membrane layer. Specifically, electrons and protons are typically generated at the anode electro-catalyst. The electrons generated must find a path (e.g., the backing layer and a current collector) through which they are transported to an external electric circuit. The protons generated at the anode electro-catalyst must be quickly transferred to the PEM layer through which they migrate to the cathode. Electro-catalyst sites are not productively utilized if the protons do not have a means for being quickly transported to the ion-conducting electrolyte. For this reason, coating the exterior surfaces of the electro-catalyst particles and/or electrode backing layer (carbon paper or fabric) with a thin layer of an ion-conductive ionomer has been used to increase the utilization of electro-catalyst exterior surface area and increase fuel cell performance by providing improved ion-conducting paths between the electro-catalyst surface sites and the PEM layer (e.g., FIGS. 1(a) and 1(b)). Such an ion-conductive ionomer is typically the same material used as the PEM in the fuel cell. An ionomer is an ion-conducting polymer. For the case of a PEM fuel cell, the conducting ion is typically the proton and the ionomer is a proton-conducting polymer. The ionomer can be incorporated in the catalyst ink (precursor electro-catalyst composition) or can be applied on the catalyst-coated substrate afterwards. This approach has been followed by several groups of researchers, as summarized in the following patents [1-9]:    1) D. P. Wilkinson, et al., “Impregnation of micro-porous electro-catalyst particles for improving performance in an electrochemical fuel cell,” U.S. Pat. No. 6,074,773 (Jun. 13, 2000).    2) J. Zhang, et al., “Ionomer impregnation of electrode substrate for improved fuel cell performance,” U.S. Pat. No. 6,187,467 (Feb. 13, 2001).    3) I. D. Raistrick, “Electrode assembly for use in a solid polymer electrolyte fuel cell,” U.S. Pat. No. 4,876,115 (Oct. 24, 1989).    4) M. S. Wilson, “Membrane catalyst layer for fuel cells,” U.S. Pat. No. 5,211,984 (May 18, 1993).    5) J. M. Serpico, et al., “Gas diffusion electrode,” U.S. Pat. No. 5,677,074 (Oct. 14, 1997).    6) M. Watanabe, et al., “Gas diffusion electrode for electrochemical cell and process of preparing same,” U.S. Pat. No. 5,846,670 (Dec. 8, 1998).    7) T. Kawahara, “Electrode for fuel cell and method of manufacturing electrode for fuel cell,” U.S. Pat. No. 6,015,635 (Jan. 18, 2000).    8) S. Hitomi, “Solid polymer electrolyte-catalyst composite electrode, electrode for fuel cell, and process for producing these electrodes,” U.S. Pat. No. 6,344,291 (Feb. 5, 2002).    9) S. Hitomi, et al. “Composite catalyst for solid polymer electrolyte-type fuel cell and process for producing the same,” U.S. Pat. No. 6,492,295 (Dec. 10, 2002).    10) B. Srinivas and A. O. Dotson, “Proton Conductive Carbon Material for Fuel Cell,” US 2004/0109816 (Pub. Jun. 10, 2004).    11) B. Srinivas, “Sulfonated Carbonaceous Materials,” US 2004/0042955 (Pub. Mar. 4, 2004).    12) B. Srinivas, “Sulfonated Conducting Polymer-Grafted Carbon Material for Fuel Cell Applications,” US 2004/0110051 (Pub. Jun. 10, 2004).    13) B. Srinivas, “Conducting Polymer-Grafted Carbon Material for Fuel Cell Applications,” US 2004/0110052 (Pub. Jun. 10, 2004).    14) B. Srinivas, “Metallized Conducting Polymer-Grafted Carbon Material and Method for Making,” US 2004/0144961 (Pub. Jul. 29, 2004).    15) B. Srinivas, “Conducting Polymer-Grafted Carbon Material for Fuel Cell Applications,” US 2004/0166401 (Pub. Aug. 26, 2004).    16) B. Srinivas, “Sulfonated Conducting Polymer-Grafted Carbon Material for Fuel Cell Applications,” US 2004/0169165 (Pub. Sep. 2, 2004).
However, this prior-art approach [1-9] of ionomer impregnation into the electrode layer and/or onto electro-catalyst particle surfaces has a serious drawback in that the ionomer commonly used as the PEM material, although ion-conducting (proton-conducting), is not electronically conducting (FIG. 1(a). This is due to the consideration that a proton-exchange membrane, when serving as the solid electrolyte layer, cannot be an electronic conductor; otherwise, there would be internal short-circuiting, resulting in fuel cell failure and possible fire hazard. Such an electronically non-conductive material, when coated onto the surface of a catalyst particle or carbon paper fiber, will render the catalyst particle or carbon fiber surface electronically non-conductive. This would prevent the electrons generated at the catalyst sites from being quickly collected by the anode electrode substrate layer and the current collector, thereby significantly increasing the Ohmic resistance and reducing the fuel cell performance. We recognized that this impregnation or coating material should not be the same ionomer used as the PEM material. Our work led to the following patent applications:    17. Bor Z. Jang, Aruna Zhamu, and Jiusheng Guo, “Process for Producing Fuel Cell Electrode, Catalyst-Coated Electrode, and Membrane-Electrode Assembly,” U.S. patent application Ser. No. 11/522,580 (Sep. 19, 2006.    18. Bor Z. Jang, Aruna Zhamu, and Jiusheng Guo, “Electro-catalyst Composition, Fuel Cell Electrode, and Membrane-Electrode Assembly,” U.S. patent application Ser. No. 11/518,565 (Sep. 11, 2006).    19. Bor Z. Jang, Aruna Zhamu, and Jiusheng Guo, “Electro-catalyst Compositions for Fuel Cells,” U.S. patent application Ser. No. 11/582,912 (Oct. 19, 2006).    20. Bor Z. Jang, Aruna Zhamu, and Jiusheng Guo, “Fuel Cell Electro-catalyst Composite Composition, Electrode, Catalyst-Coated Membrane, and Membrane-Electrode Assembly,” U.S. patent application Ser. No. 11/699,176 (Jan. 30, 2007).
In these co-pending applications [Ref. 17-20], we disclosed several new classes of electro-catalyst compositions and the processes for producing these compositions and their derived electrodes, catalyst-coated membranes (CCMs), and membrane electrode assemblies (MEAs) for PEM fuel cell applications. The electro-catalyst composition and a precursor electro-catalyst composition (e.g., ink or suspension), when used in the formation of a fuel cell catalytic electrode layer, results in a significantly improved power output. The precursor electro-catalyst composition, when deposited onto a substrate with the liquid removed, forms an electro-catalyst composition that essentially constitutes an electrode layer (a catalytic anode or cathode film). The substrate in this context can be a gas diffusion layer (carbon paper or cloth) or a PEM layer. Ultimately, the electro-catalyst is sandwiched between a gas diffusion layer and a PEM layer.
The electro-catalyst composition in the second co-pending application [Ref. 18] comprises (e.g., FIG. 4(a)): (a) a catalyst un-supported or supported on an electronically conducting carrier (e.g., carbon black particles, CB); and (b) an ion-conducting and electron-conducting coating/impregnation material in physical contact with the catalyst (e.g., this impregnation material is coated on a surface of the carrier or the catalyst particles are embedded in this impregnation material), wherein the coating/impregnation material has an electronic conductivity no less than 10−4 S/cm (preferably no less than 10−2 S/cm) and an ion conductivity no less than 10−5 S/cm (preferably no less than 10−3 S/cm). Typically, this coating/impregnation material is not chemically bonded to either the carbon black surface or the catalyst and this coating or impregnation material forms a contiguous matrix with the catalyst particles dispersed therein. This contiguous matrix, along with the conductive CB particles, forms bi-networks of charge transport paths (one for electrons and the other for protons) in a fuel cell electrode, leading to much improved fuel cell performance with much reduced resistive loss, higher catalyst utilization efficiency, and higher cell output voltage. The second co-pending application [Ref. 18] also discloses a precursor composition (e.g., an ink) that leads to the formation of the desired electro-catalyst composition or catalytic electrode by simply removing the liquid ingredient from the ink (no chemical treatment required and no chemical bonding or reaction involved).
The third co-pending application [Ref. 19] discloses another class of precursor electro-catalyst compositions that lead to the desired electro-catalyst composition by removing the liquid medium from the composition and inducing a chemical conversion or reaction of other ingredient(s) in the precursor composition. This precursor electro-catalyst composition comprises a precursor molecular metal, which can be chemically converted to nano-scaled catalyst particles via heating or energy beam exposure (e.g., UV light, ion beam, Gamma radiation, or laser beam) during or after the precursor composition is deposited with its liquid ingredient being removed. The process for producing an electrode, its CCM and MEA from this precursor electro-catalyst composition is disclosed in the first co-pending application [Ref. 17].
It may be noted that Srinivas [Ref. 10-16] prepared a group of sulfonated carbon black (CB) or conducting polymer-grafted CB particles (FIG. 2) for fuel cell applications. The sulfonated carbon material was typically obtained by reacting an anhydride with a sulfuric acid to first obtain an organic sulfate intermediate, which was then reacted with CB to impart SO3H groups to the CB. Alternatively, a multiple-step diazoitization was used to impart Φ-SO3H groups (Φ=a benzene ring). These groups were then coated with or bonded to a conducting polymer to improve the electronic conductivity of surface-treated CB particles. Further alternatively, a complex oxidative polymerization step was taken to graft a conducting polymer to CB surface, followed by sulfonation, or to obtain a grafted sulfonated conducting polymer from a sulfonated monomer [12-16]. The technology proposed by Srinivas is vastly different and patently distinct from our technology as represented by the four co-pending applications in the following ways:
(1) Srinivas's compositions are basically carbon black (CB) particles with their surfaces chemically bonded with either SO3H type functional groups or a mono-layer of conductive polymer chains. In essence, these are just surface-modified CB particles that contain a minute amount of surface functional groups and chains. In the resulting electrode, individual CB particles were being packed together but remaining as discrete particles (FIG. 1 of Ref. 12-14) in such a manner that the surface-bound chains were of insufficient amount to form a continuous matrix material of structural integrity. The requirement for these particles to strictly maintain a contiguous network is a major drawback of this prior art technology. First, it is not natural for discrete particles to form and maintain contiguity, unless the volume fraction of these particles is excessively high with respect to the surface-bound groups or chains. In such a high-loading situation (with only a small amount of surface bound groups or chains), the resulting cluster or aggregate structure is very weak in terms of mechanical strength and, hence, tends to form cracks and fail to perform its intended functions.
In contrast, the coating, impregnation or matrix material in our co-pending applications is NOT chemically bonded to either the CB surface or the catalyst. More importantly, this coating, impregnation, or matrix material serves to form a contiguous matrix with the catalyst particles dispersed therein in such a fashion that the catalyst particles or their supporting CB particles do not have to form a contiguous structure in order to maintain two charge transport paths (one for electrons and the other for proton). This matrix material is both electron- and proton-conducting anyway. When the un-supported catalysts or supported catalysts, along with the matrix material, are cast into a thin electrode layer, the matrix material automatically provides the two charge transport networks (whether the catalyst particles or CB particles form a continuous network or not).
(2) Srinivas's compositions did not include those with un-supported catalyst particles. They have essentially worked on catalysts supported on surface-grafted or -bonded CB particles only.
(3) Srinivas's compositions involved complicated and time-consuming surface chemical bonding, grafting, and/or polymerization procedures. In contrast, the compositions in our co-pending applications involve physically dispersing catalysts or carbon-supported catalysts in a fluid (a benign solvent such as a mixture of water and isopropanol in which a proton- and electron-conducting polymer is dissolved). No chemical reaction is needed or involved.(4) In the case of surface functionalization [10,11], an electronically non-conducting moiety is interposed between the CB and the conducting polymer, which could significantly reduce the local electron conductivity.(5) It is known that only a small number of functional groups can be chemically bonded to a carbon black surface and, hence, a very limited number of polymer chains are grafted to the surface. Such a surface-treated CB still has limited conductivity improvements. In fact, Srinivas could not even measure the electron and proton conductivity of these mono-layers of surface groups or grafted polymer chains. He had to mix the surface bonded CB particles with Nafion® prior to a conductivity measurement. The conductivity values obtained are not representative of the conductivities of surface-treated CB particles.(6) Although Srinivas's CB particles might be individually proton- and electron-conductive on the surface, they must cluster together to form a contiguous structure to maintain an electron-conducting path and a proton-conducting path. This is not always possible when they are used to form an electrode bonded to a PEM surface or a carbon paper surface. Due to only an extremely thin layer of chemical groups or chains being bonded to an individual CB particle, the resulting electrode can be very fragile and interconnected pores (desirable for gas diffusion) tend to interrupt their contiguity. Operationally, it is very difficult to form an integral layer of catalytic electrode from these modified CB particles alone. These shortcomings are likely the reasons why the data provided by Srinivas showed very little improvement in performance of the fuel cell featuring these coated CB particles. For instance, FIG. 8 of Ref. 10 and FIG. 8 of Ref. 12 show that the best improvement achieved by surface-bonded CB particles was a voltage increase from 0.54 V to 0.59V at 700 mA/cm2, less than 10% improvement. However, a decrease in voltage was observed at higher current densities. In contrast, our electro-catalyst compositions naturally form two charge transport paths, which are unlikely to be interrupted during the electrode formation process. We have consistently achieved outstanding fuel cell performance improvements (greater than 20% in many cases).
One special feature of the fourth co-pending invention [Ref. 20] is an electro-catalyst composite composition (e.g., FIG. 4(b)) that comprises nano-scaled catalyst particles supported on highly electron-conducting nano-scaled carbon/graphite materials such as carbon nanotubes (CNTs), nanometer-thickness graphite platelets or nano-scaled graphene plates (NGPs), carbon nano-scrolls (CNS, formed by scrolling up NGPs), carbon nano-fibers (CNFs), and graphitic nano-fibers (GNFs, which are ultra-high temperature treated CNFs). These materials exhibit an electrical conductivity that is several orders of magnitude higher than that of carbon blacks (CB). Their electrical conductivity values are also typically much higher than those of the electron- and proton-conducting matrix polymers. These elongated particles (CNTs, NGPs, CNS, CNFs, and GNFs) have an ultra-high aspect ratio (largest dimension/smallest dimension) that enable the formation of a contiguous network of electron-conductive paths with a minimum amount of particles (i.e., a very low percolation threshold). Furthermore, these elongated particles are also of high strength and stiffness and, when dispersed in a polymer matrix, significantly reinforce the structural integrity of the matrix. This is essential to the durability of an electrode in a fuel cell that is subject to thermal and humidity cycling and mechanical impacts. This is another feature that prior art compositions (including Srinivas's) do not have.
The present invention provides another class of electro-catalysts (e.g., FIG. 5) based on a proton- and electron-conducting polymer and a transition metal, in which transition metal atoms are covalently bonded to heteroatoms of the backbone monomers of the polymer. Furthermore, the covalently bonded transition metal atoms are each a nucleation site for catalytically active transition metal nano particles. These ultra-small catalyst nano particles, typically smaller than 2 nm (some as small as single atoms or just 2-20 atoms), are highly effective catalysts for both anode and cathode reactions. The backbone polymer provides the needed bi-network of electron-conducting paths and proton-conducting paths. By contrast, the conducting polymers used by Rajeshwar, et al. [Ref. 21] and Finkelshtain, et al. [22-25] are electron-conducting only, but not proton conducting. They are subject to similar shortcomings associated with the cases of coating materials based on a proton-conducting polymer only [e.g., Ref. 1-9]: i.e., only one charge-conducting network of paths, not two, was established between the gas diffusion electrode and the PEM layer.    21. K. Rajeshwar, e al, “Conductive Polymer Films Containing Nanodispersed Catalyst Particles: A New Type of Composite Material for Technological Applications,” U.S. Pat. No. 5,334,292 (Aug. 2, 1994).    22. G. Finkelsshtain, et al., “Class of Electrocatalysts and a Gas Diffusion Electrode Based Thereon for Fuel Cells,” U.S. Pat. No. 6,380,126 (Apr. 30, 2002).    23. G. Finkelsshtain, et al., “Class of Electrocatalysts and a Gas Diffusion Electrode Based Thereon for Fuel Cells,” U.S. Pat. No. 6,479,181 (Nov. 12, 2002).    24. G. Finkelsshtain, et al., “Class of Electrocatalysts and a Gas Diffusion Electrode Based Thereon for Fuel Cells,” U.S. Pat. No. 6,730,350 (May 4, 2004).    25. G. Finkelsshtain, et al., “Class of Electrocatalysts and a Gas Diffusion Electrode Based Thereon for Fuel Cells,” U.S. Pat. No. 6,878,664 (Apr. 12, 2005).
The following references appear to be somewhat relevant to the current prior art discussion:    26. Z. Qi, et al., “Electron and Proton Transport in Gas Diffusion Electrodes Containing Electronically Conductive Proton-Exchange Polymers,” Journal of Electroanalytical Chemistry, 459 (1998) 9-14.    27. K. Bouzek, et al., “Utilization of Nafion/Conducting Polymer Composite in the PEM Type Fuel Cells,” Journal of Applied Electrochemistry, 37 (2007) 137-145.    28. H. Lee, et al., “Performance of Polypyrrole-impregnated Composite Electrode for Unitized Regenerative Fuel Cell,” Journal of Power Source, 131 (2004) 188-193.
Qi, et al. [Ref. 26] suggested that conducting polymer/polyanion composite particles can be used to replace carbon black particles for supporting Pt particles. These Pt-bearing particles might be individually proton- and electron-conductive; but, they must cluster together to form a contiguous structure to maintain an electron-conducting path and a proton-conducting path. This is not always possible when they are used to form an electrode bonded to a PEM surface or a carbon paper surface. The resulting electrode can be very fragile and subject to quick failure caused by impact, vibration, or thermal cycling when a fuel cell is in use. Further, as pointed out by Qi, et al. [26], these composite particle-based catalysts tend to exhibit “inferior performance” presumably due, in part, to the Pt particles being too large (typically just under 1 μm). The work reported by Bouzek, et al [Ref. 27] and that by Lee, et al. [Ref. 28] are both based on an electron-conducting polymer (polyaniline and polypyrolle, respectively) coated on (or slightly impregnated into) a surface of a Nafion-type PEM electrolyte layer, forming a two-layer PEM structure. Bouzek, et al. [Ref. 27] showed that there was no advantage of using such a two-layer structure although Lee, et al. [Ref. 28] observed some limited improvements over conventional Nafion-supported catalyst electrode. Presumably, these electrode structures do not provide the required bi-networks of charge transport paths. They could also limit the transport of fuels (particularly when larger-than-hydrogen molecules such as alcohol are involved) at the anode and interfere with the removal of water from the cathode.
The present invention overcomes most of the drawbacks and shortcomings of the prior art electrodes by providing an innovative class of electro-catalyst composition that features a much more effective utilization of the catalyst, particularly through the optimization of electron and ion transfer rates, a significant reduction in the nano catalyst particle size, and an optimal dispersion of catalyst particles in three dimensions. The resulting electrode effectively establishes proton-conducting paths, electron-conducting paths, and fuel/oxidant transport paths that enable the same amount of electro-catalyst to induce a higher rate of electrochemical conversion in a fuel cell resulting in significantly improved performance as compared to all of the prior art electro-catalysts known to us.